For many years, solvent-cast cellulose triacetate film has been used as a photographic film support. Its physical characteristics and the dimensional uniformity and surface quality imparted by solvent casting have made cellulose triacetate the first choice for many photographic films.
Despite the excellent photographic properties of solvent-cast cellulose triacetate film, environmental concerns about solvents conventionally used in the casting of the films have created a need for a new method of manufacture of cellulose triacetate films or for a new kind of photographic film support. Unfortunately, cellulose triacetate cannot be melt-cast because its melting point is above its decomposition temperature. As for solvent casting of cellulose triacetate, no new solvents have been found which are more acceptable than the conventional ones.
One possible way to eliminate solvents is to melt cast a thermally stable polymer such as poly(ethylene terephthalate). Indeed, this type of polymer is used commercially for the manufacture of supports for photographic sheet films such as x-ray films and graphic arts films. It is not suitable, however, for many kinds of photographic films, including roll films for amateur cameras. In this use the polyester film develops curl or "core set" when wound on the film spool. Cellulose triacetate also develops curl when wound (and a certain amount of core set is desirable), but when the cellulosic film is processed in aqueous developing solutions the curl of the hydrophilic cellulosic film is relaxed and the film lies flat. Poly(ethylene terephthalate) films, on the other hand, do not relax their core set in aqueous baths so they are unsatisfactory for photographic roll films. Other polymers lack one or more of the combination of properties and capabilities that make cellulose triacetate successful as a photographic film support.
Esters of cellulose hydroxyl groups have been made over a wide range with both single and mixed acids for different uses. Cellulose diacetate (DS.sub.ac =2.45), unlike the triacetate, has a sufficiently low melting point that, with adequate plasticizer addition, it can be melt extruded. Mixed esters, or replacement of acetyl groups of the triacetate with propionic or butyric groups can accomplish the same purpose. However, films previously made from these known cellulose ester compositions of lower acetyl content than the triacetate have been deficient in properties that are necessary for photographic roll film supports, most notably in stiffness and heat distortion temperature.
In addition, the plasticizers and flame retardants previously used in melt-processable cellulose esters are volatile at extrusion temperatures and generate aerosols which can condense and create surface defects on the film. Hence a continued need exists for a melt cast film having the desired combination of properties for photographic film supports.